85.15 The site associated to a semi-representable object
Let \mathcal{C} be a site. Recall that a semi-representable object of \mathcal{C} is simply a family \{ U_ i\} _{i \in I} of objects of \mathcal{C}. A morphism \{ U_ i\} _{i \in I} \to \{ V_ j\} _{j \in J} of semi-representable objects is given by a map \alpha : I \to J and for every i \in I a morphism f_ i : U_ i \to V_{\alpha (i)} of \mathcal{C}. The category of semi-representable objects of \mathcal{C} is denoted \text{SR}(\mathcal{C}). See Hypercoverings, Definition 25.2.1 and the enclosing section for more information.
For a semi-representable object K = \{ U_ i\} _{i \in I} of \mathcal{C} we let
\mathcal{C}/K = \coprod \nolimits _{i \in I} \mathcal{C}/U_ i
be the disjoint union of the localizations of \mathcal{C} at U_ i. There is a natural structure of a site on this category, with coverings inherited from the localizations \mathcal{C}/U_ i. The site \mathcal{C}/K is called the localization of \mathcal{C} at K. Observe that a sheaf on \mathcal{C}/K is the same thing as a family of sheaves \mathcal{F}_ i on \mathcal{C}/U_ i, i.e.,
\mathop{\mathit{Sh}}\nolimits (\mathcal{C}/K) = \prod \nolimits _{i \in I} \mathop{\mathit{Sh}}\nolimits (\mathcal{C}/U_ i)
This is occasionally useful to understand what is going on.
Let \mathcal{C} be a site. Let K = \{ U_ i\} _{i \in I} be an object of \text{SR}(\mathcal{C}). There is a continuous and cocontinuous localization functor j : \mathcal{C}/K \to \mathcal{C} which is the product of the localization functors j_ i : \mathcal{C}/V_ i \to \mathcal{C}. We obtain functors j_!, j^{-1}, j_* exactly as in Sites, Section 7.25. In terms of the product decomposition \mathop{\mathit{Sh}}\nolimits (\mathcal{C}/K) = \prod \nolimits _{i \in I} \mathop{\mathit{Sh}}\nolimits (\mathcal{C}/U_ i) we have
\begin{matrix} j_!
& :
& (\mathcal{F}_ i)_{i \in I}
& \longmapsto
& \coprod j_{i, !}\mathcal{F}_ i
\\ j^{-1}
& :
& \mathcal{G}
& \longmapsto
& (j_ i^{-1}\mathcal{G})_{i \in I}
\\ j_*
& :
& (\mathcal{F}_ i)_{i \in I}
& \longmapsto
& \prod j_{i, *}\mathcal{F}_ i
\end{matrix}
as the reader easily verifies.
Let f : K \to L be a morphism of \text{SR}(\mathcal{C}). Then we obtain a continuous and cocontinuous functor
v : \mathcal{C}/K \longrightarrow \mathcal{C}/L
by applying the construction of Sites, Lemma 7.25.8 to the components. More precisely, suppose f = (\alpha , f_ i) where K = \{ U_ i\} _{i \in I}, L = \{ V_ j\} _{j \in J}, \alpha : I \to J, and f_ i : U_ i \to V_{\alpha (i)}. Then the functor v maps the component \mathcal{C}/U_ i into the component \mathcal{C}/V_{\alpha (i)} via the construction of the aforementioned lemma. In particular we obtain a morphism
f : \mathop{\mathit{Sh}}\nolimits (\mathcal{C}/K) \to \mathop{\mathit{Sh}}\nolimits (\mathcal{C}/L)
of topoi. In terms of the product decompositions \mathop{\mathit{Sh}}\nolimits (\mathcal{C}/K) = \prod \nolimits _{i \in I} \mathop{\mathit{Sh}}\nolimits (\mathcal{C}/U_ i) and \mathop{\mathit{Sh}}\nolimits (\mathcal{C}/L) = \prod \nolimits _{j \in J} \mathop{\mathit{Sh}}\nolimits (\mathcal{C}/V_ j) the reader verifies that
\begin{matrix} f_!
& :
& (\mathcal{F}_ i)_{i \in I}
& \longmapsto
& (\coprod \nolimits _{i \in I, \alpha (i) = j} f_{i, !}\mathcal{F}_ i)_{j \in J}
\\ f^{-1}
& :
& (\mathcal{G}_ j)_{j \in J}
& \longmapsto
& (f_ i^{-1}\mathcal{G}_{\alpha (i)})_{i \in I}
\\ f_*
& :
& (\mathcal{F}_ i)_{i \in I}
& \longmapsto
& (\prod \nolimits _{i \in I, \alpha (i) = j} f_{i, *}\mathcal{F}_ i)_{j \in J}
\end{matrix}
where f_ i : \mathop{\mathit{Sh}}\nolimits (\mathcal{C}/U_ i) \to \mathop{\mathit{Sh}}\nolimits (\mathcal{C}/V_{\alpha (i)}) is the morphism associated to the localization functor \mathcal{C}/U_ i \to \mathcal{C}/V_{\alpha (i)} corresponding to f_ i : U_ i \to V_{\alpha (i)}.
Lemma 85.15.1. Let \mathcal{C} be a site.
For K in \text{SR}(\mathcal{C}) the functor j : \mathcal{C}/K \to \mathcal{C} is continuous, cocontinuous, and has property P of Sites, Remark 7.20.5.
For f : K \to L in \text{SR}(\mathcal{C}) the functor v : \mathcal{C}/K \to \mathcal{C}/L (see above) is continuous, cocontinuous, and has property P of Sites, Remark 7.20.5.
Proof.
Proof of (2). In the notation of the discussion preceding the lemma, the localization functors \mathcal{C}/U_ i \to \mathcal{C}/V_{\alpha (i)} are continuous and cocontinuous by Sites, Section 7.25 and satisfy P by Sites, Remark 7.25.11. It is formal to deduce v is continuous and cocontinuous and has P. We omit the details. We also omit the proof of (1).
\square
Lemma 85.15.2. Let \mathcal{C} be a site and K in \text{SR}(\mathcal{C}). For \mathcal{F} in \mathop{\mathit{Sh}}\nolimits (\mathcal{C}) we have
j_*j^{-1}\mathcal{F} = \mathop{\mathcal{H}\! \mathit{om}}\nolimits (F(K)^\# , \mathcal{F})
where F is as in Hypercoverings, Definition 25.2.2.
Proof.
Say K = \{ U_ i\} _{i \in I}. Using the description of the functors j^{-1} and j_* given above we see that
j_*j^{-1}\mathcal{F} = \prod \nolimits _{i \in I} j_{i, *}(\mathcal{F}|_{\mathcal{C}/U_ i}) = \prod \nolimits _{i \in I} \mathop{\mathcal{H}\! \mathit{om}}\nolimits (h_{U_ i}^\# , \mathcal{F})
The second equality by Sites, Lemma 7.26.3. Since F(K) = \coprod h_{U_ i} in \textit{PSh}(\mathcal{C}, we have F(K)^\# = \coprod h_{U_ i}^\# in \mathop{\mathit{Sh}}\nolimits (\mathcal{C}) and since \mathop{\mathcal{H}\! \mathit{om}}\nolimits (-, \mathcal{F}) turns coproducts into products (immediate from the construction in Sites, Section 7.26), we conclude.
\square
Lemma 85.15.3. Let \mathcal{C} be a site.
For K in \text{SR}(\mathcal{C}) the functor j_! gives an equivalence \mathop{\mathit{Sh}}\nolimits (\mathcal{C}/K) \to \mathop{\mathit{Sh}}\nolimits (\mathcal{C})/F(K)^\# where F is as in Hypercoverings, Definition 25.2.2.
The functor j^{-1} : \mathop{\mathit{Sh}}\nolimits (\mathcal{C}) \to \mathop{\mathit{Sh}}\nolimits (\mathcal{C}/K) corresponds via the identification of (1) with \mathcal{F} \mapsto (\mathcal{F} \times F(K)^\# \to F(K)^\# ).
For f : K \to L in \text{SR}(\mathcal{C}) the functor f^{-1} corresponds via the identifications of (1) to the functor \mathop{\mathit{Sh}}\nolimits (\mathcal{C})/F(L)^\# \to \mathop{\mathit{Sh}}\nolimits (\mathcal{C})/F(K)^\# , (\mathcal{G} \to F(L)^\# ) \mapsto (\mathcal{G} \times _{F(L)^\# } F(K)^\# \to F(K)^\# ).
Proof.
Observe that if K = \{ U_ i\} _{i \in I} then the category \mathop{\mathit{Sh}}\nolimits (\mathcal{C}/K) decomposes as the product of the categories \mathop{\mathit{Sh}}\nolimits (\mathcal{C}/U_ i). Observe that F(K)^\# = \coprod _{i \in I} h_{U_ i}^\# (coproduct in sheaves). Hence \mathop{\mathit{Sh}}\nolimits (\mathcal{C})/F(K)^\# is the product of the categories \mathop{\mathit{Sh}}\nolimits (\mathcal{C})/h_{U_ i}^\# . Thus (1) and (2) follow from the corresponding statements for each i, see Sites, Lemmas 7.25.4 and 7.25.7. Similarly, if L = \{ V_ j\} _{j \in J} and f is given by \alpha : I \to J and f_ i : U_ i \to V_{\alpha (i)}, then we can apply Sites, Lemma 7.25.9 to each of the re-localization morphisms \mathcal{C}/U_ i \to \mathcal{C}/V_{\alpha (i)} to get (3).
\square
Lemma 85.15.4. Let \mathcal{C} be a site. For K in \text{SR}(\mathcal{C}) the functor j^{-1} sends injective abelian sheaves to injective abelian sheaves. Similarly, the functor j^{-1} sends K-injective complexes of abelian sheaves to K-injective complexes of abelian sheaves.
Proof.
The first statement is the natural generalization of Cohomology on Sites, Lemma 21.7.1 to semi-representable objects. In fact, it follows from this lemma by the product decomposition of \mathop{\mathit{Sh}}\nolimits (\mathcal{C}/K) and the description of the functor j^{-1} given above. The second statement is the natural generalization of Cohomology on Sites, Lemma 21.20.1 and follows from it by the product decomposition of the topos.
Alternative: since j induces a localization of topoi by Lemma 85.15.3 part (1) it also follows immediately from Cohomology on Sites, Lemmas 21.7.1 and 21.20.1 by enlarging the site; compare with the proof of Cohomology on Sites, Lemma 21.13.3 in the case of injective sheaves.
\square
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